Recently, there have been various studies and development regarding laser ignition devices to be applied to gaseous fuel engines to be used for cogeneration, and to be applied to internal combustion engines of poor ignition performance such as lead burn fuel mixture engines, etc. The laser ignition device has a semiconductor laser as an excitation light source, and oscillates excitation light and irradiates the excitation light to a laser resonator. The laser resonator oscillates a pulse laser having a high energy density on the basis of the received excitation light. A condenser unit in the laser resonator condenses the pulse laser in a fuel gas mixture introduced in the combustion chamber of the internal combustion engine so as to ignite the fuel gas mixture.
Such a laser ignition device has an ignition plug. The ignition plug has an optical element, an optical window, etc. The optical widow has heat-resistant and is arranged at a boundary between a combustion chamber and the ignition plug so as to prevent the optical element in the ignition plug from a high temperature and pressure gas in the combustion chamber. The optical element focuses the pulse laser in the inside of the combustion chamber of the internal combustion chamber so as to ignite a fuel gas mixture in the combustion chamber.
On the other hand, because the internal combustion engine uses an engine oil to reduce abrasion, etc. generated between a piston and a cylinder of the internal combustion engine, an oil mist occurs in the combustion chamber. Such oil mist floats in the inside of the combustion chamber, and is adhered on the surface at the combustion chamber side of the optical window. When a deposit is accumulated on the surface of the optical window due to the oil mist, the optical transmission properties of the pulse laser are reduced due to the deposition of such oil mist, and the presence of the deposit reduces the stable ignition capability of the ignition plug. It is accordingly desired to prevent such oil mist from being adhered on the surface at the combustion chamber side of the optical window of the ignition plug.
Further, for example, when an engine starts and an ordinary spark ignition plug operates at a low temperature and a liquid fuel is burned in incomplete combustion, soot, etc. are generated due to the incomplete combustion, and a deposit is accumulated due to such soot on a surface of an insulation glass in the ordinary ignition plug. Because the deposit is made of carbon having a conductivity, the formation of deposit reduces the electrical insulation between electrodes of the ignition plug, and deteriorates the stable ignitability of the ignition plug.
The patent document 1 has disclosed a laser-guided type external ignition plug so as to solve the conventional problem previously described. In the ignition device according to the patent document 1, a sub-chamber is formed in a combustion chamber at an end side of a combustion chamber window, and an aperture diaphragm is formed in the sub-chamber through which the laser beam passes and enters the inside of the combustion chamber through the sub-chamber. A laser beam enters the combustion chamber through the aperture diaphragm. The patent document 2 disclose an ignition plug in which an outer surface of an insulator is coated by a silicon resin.